C++ modules: if a class is defined in multiple modules (for instance, because
it is an implicit instantiation of a class template specialization), pick the
first-loaded definition to be the canonical definition, and merge all other
definitions into it.

This is still rather incomplete -- we need to extend every form of declaration
that can appear within a CXXRecordDecl to be redeclarable if it came from an
AST file (this includes fields, enumerators, ...).